In a digital transmission network, data from a large number of users are serially transmitted from one network node to another network node, up to their respective final destinations.
Due to the evolution of networks towards more and more complex mixing of sub-networks with heterogeneous architectures, it is clear that there is a requirement to support distributed computing applications across high speed backbones that may be carrying LAN traffic, voice, video and traffic among channel-attached hosts and workstations.
Fast packet switching is now commonly used to accommodate the bursty, multiprocess communication found in distributed computing environments.
Recently, the concept of cell switching has been introduced. Cell switching can be thought of as a high performance form of packet switching. In packet switching networks, the packet size is a fixed maximum, but individual packets may always be shorter than the maximum. In a cell based network, cells have a fixed length. Cells are usually a lot shorter than packets because the use of short fixed length cells simplifies the hardware needed in each node of the network.
Asynchronous Transfer Mode (ATM) is a protocol for user access to and the internal operation of a public high speed cell switching system. This protocol is suitable for all kinds of traffic: data, voice, image, video.
In order to make an ATM network practical, it is necessary to adapt the internal network characteristics to those of the various traffic types that will use the network. This is the purpose of the ATM Adaptation Layer (AAL). The function of the AAL is thus to provide generalized interworking across the ATM network. The AAL function operates an end-to-end protocol across the ATM network to provide support for end users of different classes of service corresponding to generic classes of network traffic.
One of these classes (Class One) is intended for constant rate voice and video applications. It requires a constant bit rate at source and destination, a timing relationship between source and destination and permits to transfer structured information between source and destination.
Methods that satisfy these requirements are disclosed in Revised Recommendation I.363 from CCITT, which is included herein by reference.
This Recommendation describes the interactions between the AAL and the next higher (OSI) layer and the AAL and the ATM layer (sub-layer of layer 1). The AAL isolates the higher layers from the specific characteristics of the ATM layer by mapping the higher layers Protocol Data Units (PDU) into the information field of the ATM cell and vice versa. The AAL entities exchange information with the peer AAL entities to support the AAL functions.
To support services above the AAL, some independent functions must be performed in the AAL. These functions are organized in two logical sublayers, the Convergence Sublayer (CS) and the Segmentation and Reassembly sublayer (SAR).
The SAR prime functions are segmentation of higher layer information into a size suitable for the information field of an ATM cell and reassembly of the contents of ATM cell information fields into higher layer information.
The CS prime function is to provide the AAL service at the AAL Service Access Points (SAP).
The SAR sublayer at the transmitting end accepts a 47 octet block of data (SAR.sub.-- PDU payload) from the CS sublayer and then adds a one octet SAR.sub.-- PDU header to each block to form the SAR.sub.-- PDU.
The SAR sublayer at the receiving end receives the 48 octet block of data from the ATM layer and separates the SAR.sub.-- PDU header. The 47 octet block of the SAR.sub.-- PDU payload is then passed to the CS sublayer. The basic AAL1 header is 1 octet long and the payload is 47 octets long.
FIG. 1 represents the SAR.sub.-- PDU format of AAL type 1, wherein
SN Sequence Number (4 bits) PA1 SNP Sequence Number Protection (4 bits) PA1 CSI Convergence Sublayer Indicator PA1 SNC Cell Sequence Number (0 to 7) PA1 CRC Cyclic Redundancy Check: corrects single errors PA1 P Parity: detects double errors PA1 a) setting the Convergence Sublayer Indicator (CSI) equal to 1, in the first SAR.sub.-- PDU of the connection, and PA1 b) setting an internal counter to N where N is the length of a structured data field PA1 c) setting CSI=0 in the next SAR.sub.-- PDU, and PA1 d) decrementing the internal counter by one, PA1 e) repeating steps c) and d) until the contents of the counter is equal to 1, PA1 f) setting CSI=1 in the Nth SAR.sub.-- PDU and resetting the counter to N, PA1 g) repeating steps c) to f) until the last SAR.sub.-- PDU of the connection is transmitted, PA1 h) checking that CSI =1 in the first SAR.sub.-- PDU received, PA1 i) setting the counter to N, PA1 j) checking that for the next SAR.sub.-- PDU having CSI =1 the counter is at 1, PA1 k) resetting the counter to N, PA1 l) repeating steps j) and k) until the last SAR.sub.-- PDU of the connection is received.
The format of the Structured Data Transfer (SDT) method of Recommendation I.363 is shown in FIGS. 2 and 3. These Figures represent the format of the SAR.sub.-- PDU payload. The SAR.sub.-- PDU payload used by the CS has two formats, called non-P (FIG. 2) and P (FIG. 3) format. The CS procedure for SDT uses a pointer to delineate the structure boundaries. The pointer field contains the binary value of the offset, measured in octets, between the end of the pointer field and the first start of the structured block in the 93 octets payload consisting of the remaining 46 octets of this SAR.sub.-- PDU payload and the 47 octets of the next SAR.sub.-- PDU payload.
This method has many disadvantages. The SAR.sub.-- PDU header length and the SAR.sub.-- PDU payload length are variable. When one cell is lost, the receiver needs to know if the cell contained 46 or 47 octets of user information. The processing is complex. It requires reading of the pointer of P-formats and identification of the start of the structured data field.
An alternative is described wherein the cells are partially filled. The payload is partially filled with user data. The user data length is of N octets or a multiple of N octets. It guarantees that the first octet of the payload is also the first octet of the structured data field. The format of the partially filled cell SAR.sub.-- PDU payload is shown in FIG. 4.
The disadvantages of the method are that it does not apply for structured data field lengths greater than 47 octets and that the dummy data transported in each cell reduces the efficiency of the ATM connection.